1. Field of the Invention
The present invention relates to a core structure of a heat exchanger having tubes through which a heat exchange medium flows being fixed to seat plates and corrugated fins radiating heat of the heat exchange medium through the tube, especially the core structure used for a heat exchanger such as a radiator for a motor vehicle or the like.
2. Description of the Related Art
A conventional core structure of a heat exchanger is, for example, disclosed in Japanese Patent Laid-open No. Tokkaihei 11-14285 and in Japanese Patent Laid-open No. Tokkaihei 9-318292. These conventional core structures of the heat exchangers have structures in which both edge portions of seat plates arranged opposite to each other are coupled by reinforcements.
FIG. 8 shows an example of the conventional core structure of the heat exchanger, in which tubes 102 and corrugated fins 103 are arranged alternately between seat plates 101 arranged opposite to each other with a predetermined space interposed therebetween, and both edge portions of the seat plates 101 are coupled and reinforced by reinforcements 104.
On the seat plates 101, as shown in FIG. 9, tube holes 105 for fixing the tubes 102 by insertion and connection portions 106 having wall portions with tube holes 105 projecting to extend along the tubes 102 are formed by burring.
However, in the conventional core structure of the heat exchanger, when coolant flowing from an engine into a radiator rapidly changes in temperature from low to high, large thermal expansion of the tubes 102 and the seat plates 101 occurs, which may cause the connection portions 106 to press the tubes 102 to crack and/or break root portions of the tubes 102.
Incidentally, the rapid change of coolant flowing from the engine into the radiator in temperature from low to high occurs, for example, in a case that when the engine is started in a cold region, coolant of the engine increases gradually in temperature but does not flow into the radiator until it reaches a valve-opening temperature of a thermostat, and then the temperature of the coolant becomes high to cause a valve of the thermostat to open, so that the coolant of high temperature flows into the radiator for the first time, or in a case of, what is called, hunting phenomenon such that the thermostat repeats opening and closing while driving in the cold region.
On the other hand, as shown in FIG. 10, as the tubes 102, flat tubes having partitions 104 inside, as disclosed in Japanese Patent Laid-open No. 2002-303496 for example, have become the mainstream in recent years. However, due to the partitions 104 formed inside, the flat tubes 102 have a small allowable amount of deformation against an external pressure, so that the alleviation of thermal stress of the seat plates 101 against the tubes 102 has been an urgent issue.
The present invention has been made in light of the above-described problems, and an object thereof is to provide a core structure of a heat exchanger which is capable of preventing a crack and a breakage of root portions of tubes fixed to seat plates due to thermal stress of the seat plates against the tubes when coolant flowing from an engine into a heat exchanger, such as a radiator, rapidly changes in temperature from low to high.